-Attack damage is the only important offensive stat for many of these champions.
-Cooldown reduction gets better as you build more. That is why previous sources of cooldown reduction were unique, and the stat has a hard cap in the first place.
-You can never have too much health.
-Armor penetration has become a lot better.

The build is trolly, but it is not weak or stupid as previous stacking builds have been. Black Cleaver stacking is a potential problem, and it may very well be an optimal build for some ad ratio champions.